Combustor flow controller for gas turbine

ABSTRACT

A flow controller ( 5 ) for supplying air to a combustor and a combustor incorporating the same are disclosed. The flow controller comprising a conduit ( 6, 7, 8 ) and a control port ( 9 ), the conduit including a main section ( 8 ) dividing into at least two secondary sections ( 6, 7 ) at a junction and the control port being positioned in the conduit adjacent to the junction. In one embodiment, the control port is connected to a reservoir ( 10 ) wherein, in use, a change in the flow rate of a main airflow flowing through the main section of conduit causes a control airflow to flow either into or out of the control port whereby the main airflow is selectively diverted into one or other of the secondary sections of conduit. The main airflow may coanda around a surface of the main section. The control port may also be connected to the conduit further upstream of the junction so as to form a control loop ( 16 ). The main section of conduit may comprise a convergent-divergent duct.

BACKGROUND OF THE INVENTION

1. Filed of the Invention

This invention relates to improved combustor arrangements for gasturbine engines and in particular is concerned with control of air flowto combustor zones.

2. Discussion of Prior Art

The invention relates to improved combustor arrangements for gas turbineengines and in particular is concerned with control of air flow tocombustor zones.

Gas turbine engines include an air intake through which air is drawn andthereafter compressed by a compressor to enter a combustor at one ormore ports. Fuel is injected into the combustion chamber by means of afuel injector whence it is atomised, mixed with the compressed air fromthe various inlet ports and burnt. Exhaust gases are passed out of anexhaust nozzle via a turbine which drives the compressor. In addition toair flow into the combustion chamber through the air inlet ports, airalso enters the combustion chamber via the fuel injector itself.

Conventional combustors take a variety of forms. They generally comprisea combustion chamber in which large quantities of fuel are burnt suchthat heat is released and the exhaust gases are expanded and acceleratedto give a stream of uniformly heated gas. Generally the compressorsupplies more air than is needed for complete combustion of the fuel andoften the air is divided into two or more streams, one stream introducedat the front of the combustion chamber where it is mixed with fuel toinitiate and support combustion along with the air in the fuel airmixture from the fuel injector, and one stream is used to dilute the hotcombustion product to reduce their temperature to a value compatiblewith the working range of the turbine.

Gas turbine engines for aircraft are required to operate over a widerange of conditions which involve differing ratios between the massflows of the combustion and dilution air streams. To ensure a highcombustion efficiency, it is usual for the proportion of the totalairflow supplied to the burning zone to be determined by the amount offuel required to be burned to produce the necessary heat input to theturbine at the cruise condition. Often the chamber conditions arestoichiometric in that there is exactly enough fuel for the amount ofair; surplus fuel is not completely burnt. However because ofvariability of the cycles and because air and fuel are never completelymixed there are always some oxides of nitrogen and unburnt fuelresidues. An ideal air fuel mixture ratio at cruise usually leads to anover rich mixture in the burning zone at high power conditions (such astake-off) with resultant unburnt hydrocarbon and smoke emission. It ispossible to reduce smoke emission at take-off by weakening the burningzone mixture strength but this involves an increase in primary zone airvelocity which makes ignition of the engine difficult to achieve,especially at altitude.

The temperature rise of the air in the combustor will depend on theamount of fuel burnt. Since the gas temperature required at the turbinevaries according to the operating condition, the combustor must becapable of maintaining sufficient burn over a range of operatingconditions. Unwanted emissions rise exponentially with increase intemperature and therefore it is desirable to keep the temperature low.With increasingly stringent legislation against emissions, enginetemperature is an increasingly important factor, and operating thecombustor at temperatures of less than 2100 K becomes necessary. Howeverat low temperatures, the efficiency of the overall cycle is reduced.

It is a requirement that commercial airliners can decelerate rapidly inthe case of potential collision. In order to decelerate a gas turbinefrom high power to low power, the fuel flow to the engine is reduced.Although the reduction in fuel flow is almost instantaneous, the rate ofreduction of engine airflow is relatively slow because of the inertia ofrotating parts such as turbines, compressors, shafts etc. This producesa weak mixture of fuel and this increases the risk of flame extinction.It is not always easy to relight the flame especially when the combustoris set to run weakly and at high altitude. Because modern combustorsinvariably operate in lean burn principles in order to reduce oxide ofnitrogen emissions, combustors need to be operated as close to the leanextinction limit at all engine operating conditions. If margins are setwide enough to prevent flame extinction then emissions performance iscompromised.

Combustion is initiated and stabilises in the pilot zone, the mostupstream section of the combustor. Low power stability requires richareas within the primary zone of the combustor, enabling combustion tobe sustained when the overall air/fuel ratio is much weaker than theflammability limit of kerosene. In traditional combustion systems richregions can occur in the combustor due to poor mixing and pooratomisation resulting in large droplets of fuel being formed.

Conventional gas turbine engines are thus designed as a compromiserather than being optimised, because of consideration of the abovementioned conflicting requirements at different operating conditions.New “staged” design of combustors overcome the problems to a limitedextent. These comprise two combustion zones, a pilot zone and a mainzone, each having a separate fuel supply. Essentially this type ofcombustor is designed such that a fixed flow of about 70% enters thecombustor at the main zone and about 30% of the air flows to the pilotzone. In such systems the air/fuel ratio is determined by selecting theamount of fuel in each stage. The air/fuel ratio governs the temperaturewhich determines the amount of emissions. Current gas turbine enginetrends are towards increased thrust/weight ratios which require theengine to perform at higher operating compression ratios and widerranges of combustor air/fuel ratios. Future gas turbine combustionsystems will be expected to perform at higher inlet temperatures andricher air/fuel ratios. Because there is little variability in theairflow proportions to the main stage and pilot stage the amount ofoptimisation achievable for each operating condition is reduced. Eventhese combustor designs will suffer from either high nitrogen oxide andsmoke emissions at full power, or poor stability at low power.

It is therefore desirable to improve control of the amount of fuel, airand air/fuel ratio in each combustor zone to reduce the problems of weakflame extinction, emissions of oxides of nitrogen and unburnt fuel atall operating conditions, whilst maintaining good efficiency andperformance.

Conventionally, as shown in GB 785,210, this can be achieved bydiverting a main airflow flowing through a main conduit into one of twosubsidiary conduits by injecting under pressure into the main airflow acontrolling air stream. However, this requires a separate compressorwhich is disadvantageous in terms of cost and weight. Alternatively, GB1,184,683 discloses a system whereby a suction action is utilised.However, this is achieved by bleeding compressed air out of the engineresulting in a loss of engine efficiency.

It is an objection of the invention to provide enhanced means by whichair flow can be controlled.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, a flow controllerfor supplying air to a combustor comprises conduit and a control port,the conduit including a main section dividing into at least twosecondary sections at a junction characterised in that the control portis positioned in the conduit adjacent to the junction and connected to areservoir; and wherein, in use, a change in the flow rate of a mainairflow flowing through the main section of conduit causes a controlairflow to flow either in to or out of the control port whereby the mainairflow is selectively diverted into one or other of the secondarysections of conduit.

A change in the flow rate of a main airflow results in a change in thestatic pressure of the main airflow which produces a pressuredifferential between the conduit adjacent to the port and the reservoir.The pressure differential causes the control airflow until pressureequalisation, the duration of the flow depending, amongst other things,on the size of the reservoir.

In an alternative embodiment, a flow controller for supplying air to acombustor comprises conduit and a control port, the conduit including amain section dividing into at least two secondary sections at a junctioncharacterised in that the control port is positioned in the conduitadjacent to the junction; and wherein, in use, a control airflow flowingeither in to or out of the control port causes a main airflow flowingthrough the main section of conduit to coanda around a surface of themain section whereby the main airflow is selectively diverted into oneor other of the secondary sections of conduit. Ideally, the flowcontroller comprises at least one arcuate surface common to both themain section and a secondary section.

A skilled person would interpret coanda in relation to the coandaeffect, the coanda effect being the tendency of a fluid jet to attach toa downsteam surface roughly parallel to the jet axis. If this surfacecurves away from the jet the attached flow will follow it deflectingfrom the original direction (Dictionary of Science and Technology,Larousse 1995).

Preferably, the control port is connected to the conduit furtherupstream of the junction so as to form a control loop.

In a further embodiment, a flow controller for supplying air to acombustor comprises conduit and a control port, the conduit including amain section dividing into at least two secondary sections at a junctioncharacterised in that the control port is positioned in the conduitadjacent to the junction and connected to the conduit further upstreamof the junction so as to form a control loop; and wherein, in use, acontrol airflow flowing either in to or out of the control port causes amain airflow flowing through the main section of conduit to beselectively diverted into one or other of the secondary sections ofconduit.

Preferably, the main section of conduit comprises a convergent-divergentduct; wherein, in use, the control airflow flowing either in to or outof the control port is caused by a pressure differential across theduct.

According to a second aspect of the present invention, a gas turbinecombustor comprises a flow controller as described above. Ideally, theflow controller comprises two secondary sections of conduit connected totwo different zones within the combustor. In a preferred embodiment, theflow controller comprises one secondary section of conduit connected toa pilot combustion zone within the combustor and another secondarysection of conduit connected to a main combustion zone.

In this way the proportion of flow to the main combustor zone and thepilot zone can be selectively altered without mechanical means. Thisprovides robust control of flow with high reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

A combustor incorporating a flow controller according to the presentinvention will now be described, by way of example only, with referenceto the drawings of which:

FIG. 1 shows a schematic sectional view of a combustor incorporating aflow controller of the present invention;

FIG. 2 shows the combustor of FIG. 1 in greater detail;

FIGS. 3a to d show the operation of the flow controller of the combustorof FIG. 1 at various operating conditions.

FIG. 4 shows alternative embodiments of the flow controller comprisingone or more control ports in various locations.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

FIG. 1 shows a schematic view of a combustor incorporating a flowcontroller of the present invention. The combustor 1 comprises a main(high power) combustor zone 2 and pilot (low power) 3 combustor zone.Attached to the pilot zone is a primary fuel injector 4. Air flow intothe combustor enters the through a common entry point and a flowcontroller 5 which subdivides into two conduits one, 6, which leads tothe main zone and the other, 7 to the pilot zone.

FIG. 2 shows the flow controller for the combustor in more detail. Thefigure also shows a series of planes P1 to P4, in order to assist in thedescription of the flow controller. The air supply to the combustor isfrom a flow controller which comprises a main conduit 8 which dividesinto two separate sub conduits at P3, of which one (6) enters the maincombustion zone, and the other (7) enters the pilot combustion zone.Upstream of the divergence formed by the subdivision of the conduit islocated a control port 9. Port 9 is connected to a reservoir 10 whichincludes a valve 11 located on the other side which connects to the samepressure as at P1. A pressure difference exists from P1 to P4 such thatair flows from P1 to P4. The conduit from P1 to P3 acts as a venturi.From P1 to P2 the flow cross section is such that flow of airaccelerates and the static pressure falls to P2 which is lower than P1.This ensures that when valve 11 is open air will flow into the devicefrom the control loop 16 and the control port. Downstream of P2 is adiffuser.

The angle of the diffuser is sufficiently large such that flow willcoanda or attach to one or other of the outer walls. Some degree ofdiffusion and pressure recovery will take place and is essential inorder for flow acceleration and pressure reduction at plane 2.

The operation of the embodiment described above will now be describedwith reference to FIG. 3. FIG. 3a shows the operation at idle condition.The reservoir pressure is neutral and the valve is opened such thatcontrol flow is injected through control port into the main flow whereit acts as a boundary layer trip such as the main flow separates fromwall to wall. The air flow now flows through sub conduit 6 to the mainzone of the combustor. FIG. 3b shows that on acceleration, main flow isswitched back to the sub conduit which leads to the pilot zone of thecombustor by shutting the valve 11. Control flow is sucked into thecontrol port because the reservoir pressure is low relative to thepressure at P1. FIG. 3c shows that at cruise condition the valve remainsshut and the reservoir pressure is neutral. Air continues to flow to thepilot zone. On deceleration (FIG. 3d) the reservoir pressure isoverpressurised and flow out of the control port causes the main flow todivert into the conduit to the main zone.

The above described embodiment describes how control flow through a portin the flow controller can selectively divert flow, and flow control ofair to each combustor zone is automatically selected.

In a simple embodiment of the invention, the control flow loop whichincludes the reservoir and valve is dispensed with. Selectiveover-pressure or under-pressure at the control port will enableselective diversion of flow air to the respective combustor zones.

In the embodiment only one control port is described. However any numberof control ports in the vicinity of the divergence will have acontrolling effect to direct the main air flow. FIG. 4 shows fourpossible locations of control ports. Over-pressure (flow into conduit)at any of ports 12 or 14 will tend to divert flow to the sub-conduit 7and conversely underpressure at any of ports 13 or 15 will tend todivert the flow to this sub-conduit.

Overpressure at any of ports (flow to main conduit) 13 or 15 will divertflow to the sub-conduit 6 and conversely under-pressure in any of ports12 or 14 will tend to divert the flow to the sub-conduit 7.

The flow controller may contain any number of control ports whichsupplement each other, for example, a feedback loop comprising a valveof reservoir positioned between a port in the sub-conduit 14 and a portin the subconduit 12 whereby, the diversion of flow, say from subconduit7 to subconduit 6, is rendered temporary. This is particularly usefulfor temporary diversion of an airflow to a main combustor zone rather apilot combustor zone of a combustor such that during sharp deceleration,flame extinction is prevented.

The diverted flow is stable in either of the two states even if there isno applied control flow. However the control flow is preferably providedby selective over-(or under-) pressure at one of two ports 12, 13oppositely located adjacent the respective sub-conduit.

What is claimed is:
 1. A flow controller for supplying air to a turbineengine combustor said controller comprising: a conduit carrying engineairflow, the conduit including a main section dividing into at least twosecondary sections at a junction, said at least two secondary sectionscomprising main and pilot sections; a control port, said control portpositioned in the conduit adjacent to the junction; a reservoir fluidlyconnected to said control port, wherein an increase and decrease in theflow rate of a main airflow through the main section of conduit causes acontrol airflow to flow out of and into, respectively, the control portselectively diverting said main airflow into said secondary pilot andsaid secondary main sections, respectively.
 2. A flow controller forsupplying air to a turbine engine combustor, said controller comprising:a conduit carrying engine airflow, the conduit including a main sectiondividing into at least two secondary sections at a junction, said atleast two secondary sections comprising main and pilot sections; and acontrol port, said control port positioned in the conduit adjacent tothe junction wherein a control airflow flowing out of and into,respectively, the control port causes a main airflow flowing through themain section of conduit to coanda around a surface of the main sectionwhereby the main airflow is selectively diverted into said secondarypilot and said secondary main sections, respectively.
 3. A flowcontroller according to claim 2 wherein the flow controller comprises atleast one arcuate surface common to both the main section and one ofsaid at least two secondary sections.
 4. A flow controller according toclaim 1 wherein the control port is connected to the conduit upstream ofthe junction so as to form a control loop.
 5. A flow controller forsupplying air to a turbine engine combustor, said controller comprising:a conduit carrying engine airflow, the conduit including a main sectiondividing into at least two secondary sections at a junction, said atleast two secondary sections comprising main and pilot sections; and acontrol port, said control port positioned in the conduit adjacent tothe junction, wherein the control port is connected to the conduitupstream of the junction so as to form a control loop, wherein a controlairflow flowing out of and into, respectively, the control port divertssaid engine airflow into said secondary pilot and said secondary mainsections, respectively.
 6. A flow controller according to claim 4wherein the main section of conduit comprises a convergent-divergentduct; wherein, the control airflow flowing through the control port iscaused by a pressure differential across the duct.
 7. A gas turbinecombustor comprising: a gas turbine engine combustor having at least twodifferent secondary zones; and a flow controller according to claim 1wherein said secondary main section is fluidly connected to one of saidat least two different secondary zones and said secondary pilot sectionis fluidly connected to the other of siad at least two differentsecondary zones.
 8. A gas turbine combustor according to claim 7 whereinthe secondary main and pilot sections of conduit connected to main andpilot zones, respectively, within the combustor.
 9. A gas turbinecombustor according to claim 7 wherein the flow controller comprises onesecondary section of conduit connected to a pilot combustion zone withinthe combustor and another secondary section of conduit connected to amain combustion zone within the combustor.
 10. A flow controller forsupplying air to a turbine engine combustor where said combustor hasmain and pilot sections, said turbine engine having a source of highpressure air, said controller comprising: a conduit from said source ofhigh pressure air to said combustor, said conduit including an upperstream main section and, at a junction, a down stream section dividedinto a main section and a pilot section for providing airflow to saidmain and pilot sections, respectively, said main section and saidjunction comprising a structure for maintaining airflow substantially inone of said main section and said pilot section unless diverted; acontrol port, said port positioned in said main section adjacent to thejunction, for changing airflow between said main section and said pilotsection; and a reservoir fluidly connected to said control port,wherein, during acceleration of said engine, an increase in main sectionair flow rate causes a control airflow to flow into the control port andthe reservoir, diverting main section airflow at said junction to saidpilot section, and during deceleration of said engine, a decrease inmain section air flow rate causes a control outflow out of the controlport and the reservoir, diverting main section airflow at said junctionto said main section.
 11. The flow controller in accordance with claim10, wherein said reservoir has a fixed volume during idle, acceleration,cruise, and deceleration phases of engine operation.
 12. The flowcontroller in accordance with claim 11, wherein said reservoir includesa valve which, during engine start operation, is at least partially openand supplies high pressure air to said reservoir causing said controloutflow out of the control port and the reservoir, and diverting mainsection airflow at said junction to said main section.